Transfer molding refers to molding with thermosetting plastic materials, referred to as molding compounds, in which the plastic is softened by heat and pressure in a transfer chamber, then forced at high pressure through suitable sprues, runners, and gates into a closed mold for final curing. Transfer molding was invented in the 1920's by the Shaw Company. By the 1950's transfer molding had become generally utilized in the encapsulation of electronic devices such as diodes, rectifiers and switches in plastic. By the early 1960's transfer molding was utilized to encapsulate semiconductor devices such as transistors.
U.S. Pat. No. 3,367,025 to Doyle, describes a method for encapsulating a semiconductor device in plastic by transfer molding. The particular semiconductor device disclosed is a planar transistor. In the method disclosed in the '025 patent the planar transistor device is mounted on a flattened portion of the base lead. Electrical connections are made between the device and the emitter and collector leads of the finished transistor using extremely fine gold wire, typically between 3-5 mils in diameter. The three leads are then held by a portion of a molding jig that also places the device and electrical connections in the interior of a mold cavity. Plastic is injected into the mold cavity through a gate located in the top half of the mold cavity, but beneath the device and the electrical connections, to encapsulate the device.
U.S. Pat. No. 3,439,238 to Birchler et al. also discloses a process for encapsulating a transistor in plastic. In the method disclosed in the '238 patent transistor leads are disposed at the parting line of the two halves of a mold cavity with the transistor device and the electrical connections being disposed in the top half of the cavity. To encapsulate the device, plastic is injected through a gate located in the bottom half of the mold cavity, i.e. on the opposite side of the mold cavity from the device and electrical connections. The transistor is held firmly in place by supporting it on a conductor held at both ends. The '238 patent discloses that locating the gate on the opposite side of the mold cavity from the device and whisker wire connections helps to prevent direct contact between the injected plastic and the device and wires. According to the patent, the avoidance of avoid direct contact helps to prevent the plastic from dislodging or breaking the electrical connections. Injecting the plastic into the bottom half of the mold cavity also allows the mold to be more easily cleaned as the hardened plastic around the gate, and the runner system of the mold, can be easily seen by the mold operator. Additionally, upon lifting the top half of the mold the device may be viewed. Two additional related patents, U.S. Pat. Nos. 4,043,027 and 3,716,764, were issued based on the same disclosure as the '238 patent.
Today, transfer and injection molding are widely utilized to encapsulate electronic components, particularly semiconductor devices, in plastic. In generally utilized processes, a semiconductor die, generally an integrated circuit wafer, is mounted on a leadframe. Electrical connections are made from terminals on the die to leads on the leadframe to provide external connections for the encapsulated device. The electrical connections usually comprise 1-3 mil in diameter gold wires which are connected to the device and the leadframe in an automated wire bonding process. The electrical connections are sometimes referred to as bond wires, or whisker wires.
After the electrical connections have been made, the leadframe, which may contain many active elements, is placed in a recess in one half of a two part separable mold. Each half of the mold has a stiff backplate which is mounted on a platen of a hydraulic or mechanical press. The mating surfaces of the mold are often referred to as the parting line.
For insertion of the leadframe the two halves of the mold are held apart. The mold press is then activated to close the two mold halves together at the parting line, forming a cavity around each semiconductor die. The separable mold halves press tightly against the leadframe around the perimeter of each cavity in order to seal each cavity. Runners and channels connect each cavity to one or more central reservoirs or pots in which plastic is placed. The plastic may be preheated in the reservoir or pot to soften the plastic. A hydraulically or mechanically driven ram compresses the plastic in the reservoir or pot so that it flows through the channels and runners to each mold cavity. The plastic proceeds to enter the mold cavity where it hardens to encapsulate the device and electrical connections. Typically a thermosetting plastic is utilized. Various types of thermosetting plastics are well known to those skilled in the art.
In order to push liquified plastic from the reservoir(s) into the mold cavities it is frequently necessary to inject or transfer the plastic at pressure exceeding 1000 pounds per square inch. The point at which plastic enters each mold cavity is generally referred to as the gate. The transfer pressures utilized cause the heated plastic to enter the cavity at high velocity. The result is a region of high velocity plastic flow near the gate that dissipates as the plastic moves into the cavity and assumes a plug flow configuration.
The shape of the mold cavity, and the configuration of the leadframe, will determine the shape of the final plastic package. Commonly utilized plastic package types include: the PDIP (plastic dual in line package), the PLCC (the plastic leaded chip carrier), the PQFP (plastic quad flat pack), the SOIC (small outline integrated circuit) and the SOJ (small outline J lead). These packages are generally produced to meet JEDEC size and outline specifications.
Although this type of a molding process works reasonably well, some shortcomings do exist. As an example, during encapsulation the encapsulating material can cause the thin connecting wires on the semiconductor device to be dislodged and/or to move into contact with one another. These conditions are generally collectively referred to as wire sweep. It appears that wire sweep may result from the failure of the encapsulating material to be at the proper temperature during the molding operation. This causes the plastic to be insufficiently fluidized during injection into the cavity. Injecting the plastic under too high a pressure, or too great a flow rate, may also cause wire sweep. Thus, in encapsulation processes generally utilized today, the temperature of the mold and the pressure and transfer speed at which the plastic is introduced are carefully controlled. It is also recognized that the size and shape of the runners and gates in the mold also affect the pressure and rate at which the plastic enters the individual mold cavities.
Another problem with generally utilized molds and molding processes is the occurrence of flash. Flash refers plastic, typically in the shape of thin sheets or webs, which forms between the mold halves or mold parts, and/or on the leads in locations where no plastic is desired. Flash is undesirable since additional effort must be expended to remove it from the leadframes after molding and to clean away fragments which may have stuck to the mold or dropped onto the mold during unloading of the leadframes. If the flash is not removed from the mold prior to the next shot, then hardened flash may cause coining of the mold and shorten its working life. The problem of flash is most troublesome in the portions of the mold surrounding the leadframe regions, including the sealing surfaces and various edges, as well as near the gate.
Still another problem in molding processes is the occurrence of voids in the encapsulating material itself, thereby not providing ideal encapsulation. Voids can refer both to holes in the hardened plastic package, or an incomplete package. It is believed that the voids are sometimes caused by the lack of fluidization of the encapsulating material, or by a chase or runner jamming mechanism which occurs when the encapsulating material starts to solidify before it reaches the mold cavity. As additional encapsulating material is forced down the runners the partially solidified material enters the cavity but fails to completely fill the cavity.
In generally utilized processes, the semiconductor device and electrical connections between the device and the leadframe are located in the top half of the mold and the gate is located in the bottom half of the mold. This configuration has been referred to as opposite-side gating. More generally, opposite-side gated process is used to refer to a process wherein the gate in the mold is located on the opposite side of the leadframe from the device and electrical connections. Thus, if during molding the gate is located in the top half of the mold, above the leadframe, in an opposite-side gated process, the device and electrical connections are located in the bottom half of the mold, below the leadframe.
For a variety of reasons it may be desirable to transfer mold utilizing a process wherein the gate is located on the same side of the leadframe in the mold as the semiconductor device and the electrical connections. This type of process is referred to as a same-side gated process. If the gate is located in the top half of the mold cavity above the leadframe, the device and electrical connections are also located in the top half of the mold cavity above the leadframe. Similarly, if the gate is located below the leadframe in the bottom half of the mold cavity, the device and electrical connections are also located below the leadframe.
For many package types, in a same-side gated process the electrical connections between the device and the leadframe are located closer to the gate then they would be in an opposite-side gated process. The closeness of the gate to the electrical connections can result in severe wire sweep problems. Conceptually, the technique for solving the wire sweep problem would be to fill the cavity as slow as possible, i.e. reduce the velocity of the injected plastic, to avoid disturbing the electrical connections. However, slower transfer times may cause polymerization or gelling of the plastic mold compound prior to its completely filling the mold cavities. This would manifest itself as voids in the packages. Additionally, slower transfer times might also lead to wire sweep problems as the thickening plastic could dislodge electrical connections.
We have discovered a same-side gated process for encapsulating semiconductor devices that overcomes these problems.